What is this gun??

[Bill Murray]

From an ID contest on another forum. I have spent four hours googling and come up with nothing.

The hints given are that the name of the gun starts with "m" and is also the name of a famous warship.

Supposedly it was an experiment carried out in 1943 and would seem to be either a 17 pounder or a 76mm gun. Oh...it is British.
Also supposedly used a Bedford engine for mobility to move it around once in place.

Any arty experts on this forum??
Bill